1. Field of the Invention
The invention relates to the detection and the protection of an electrical fault and more particularly of an electrical fault in an electrical distribution box. The invention applies advantageously but in a non-limiting manner to electrical distribution in an aircraft.
2. Description of the Relevant Art
The distribution of electrical energy in an aircraft is very hierarchical. Power is firstly produced by means of generators driven by the engines of the aircraft. Then, this energy is conveyed to primary distribution boxes. The electrical energy is then distributed by cables exiting the primary distribution boxes either directly to loads, or to secondary distribution boxes which themselves supply the remainder of the electrical loads.
This operating principle could be modified when one of the generators dedicated to a primary distribution box is inoperative. In this case, another generator of the airplane will be able to replace the generator which is inoperative.
A first type of protection, called overload protection is known from the prior art. This first type of protection comprises a command means for the generator, a line contactor placed on the cables which exit the primary and secondary distribution boxes and a control means for the line contactors. These protection elements allow shutdown of the generator and opening of the line contactors in case of a current overload or a fault. This first type of protection can also comprise at the output of the distribution boxes, instead of or as a supplement to the line contactor, a circuit breaker, a fuse or a circuit breaker contactor. These protection elements also allow opening in case of a current overload or of a fault on the cable that they are protecting.
To allow selectivity, that is to say shutdown of electricity distribution at a necessary and sufficient rank of the distribution, the protection elements mentioned hereinabove have current-detection thresholds and tripping times which differ. Their tripping time depends especially on their rank in the distribution. For example, the tripping time of a fuse of a cable at the output of a secondary distribution box will be less than the tripping time of the generator command means. This makes it possible for just the protection element located directly upstream of the electrical fault to trip, the protection elements further upstream not tripping. That said, for example in the case of the generator command means, its tripping time may be as long as 5 seconds. Thus, if a fault appears in the primary distribution box, it will be necessary to wait nearly 5 seconds before shutdown of the generator. This time is long enough to cause damage.
A second type of protection, called differential protection, which comprises a measurement of the current upstream and downstream of a distribution line, is also known from the prior art. In the case of absence of fault, the two measurements of current are equal. In the case of a fault, the current is different and this fault will be easy to locate since it is located between the two measurement points. This protection is for example used to protect the cables of the generator. On the other hand, this protection cannot easily be used to detect an internal short-circuit of a distribution box since a measurement is then necessary on each of the distribution lines exiting from the distribution box. Considering the number of lines to be measured and the disparity of the sensors used, there exist high risks of untimely tripping of the protection.